According to conventional water supply systems, when a person wants to use hot water from a point-of-use (e.g. faucet tap), the person may need to wait about 45 seconds before water coming out of the point-of-use becomes hot. This is because there may be cold water in a water pipe between a water heater that heats the water and the point-of-use, and hot water can reach the point-of-use only after the cold water in the water pipe is replaced by the hot water.
There are three main disadvantages of the conventional water supply systems. Firstly, it may take about 45 seconds to wait for the hot water to come out of the point-of-use, Secondly, each time a person waits for the hot water to come out of the point-of-use, there may be about 4 L of water that is wasted. Thirdly, the water pipe heats up from the hot water running through the water pipe only to cool down after use, which results in wasted heating energy by the water heater.
A household with a family of three people using a conventional water supply system might for example spend a total of 46 hours in a year waiting for hot water, which may waste 14,600 L of water and 238 kWh of energy for the year. Thus, it is clear that conventional water supply systems have drawbacks.
An existing approach is to use a hot water circulation pump to accelerate the speed of hot water supply to the point-of-use, or use a circulation pump to maintain a higher water temperature in the water pipe in order to reduce the time spent waiting for the hot water. Compared to the conventional water supply systems, the hot water circulation pump saves water. However, this does not avoid wasting heating energy because the water pipe is heated only to lose heat to its surroundings.
Another existing approach is to install an instant water heater at the point-of-use. However, the instant water heater uses a power supply that must be very powerful to quickly heat up water. For example, the power supply may need to deliver more than 3 kW. Additional electric wire installation may be needed when using electricity, or gas pipes may need to be installed when using gas.
Another existing approach is to install a hot water tank at the point-of-use. However, to physically fit the hot water tank at the point-of-use (e.g. under a sink in a bathroom), the hot water tank may be relatively small, which may limit a capacity of hot water that can be delivered. Thus, the hot water tank may not be able to provide hot water for a long period of time or for frequent uses.